1. Field of the Invention
The present invention relates to a lithium ion secondary cell, and more specifically to a positive electrode active material for a lithium ion secondary cell.
2. Description of the Related Art
Lithium ion secondary cells are low in weight and make it possible to obtain a high energy density. For this reason, such cells have been widely used as portable power sources for personal computers, portable terminals, etc. and also as drive power sources for vehicles.
Among these applications, lithium ion secondary cells that are used as drive power sources for vehicles need to be charged or discharged at a high current (high rate) over a very short period of time. Therefore, as one of the performances required in particular for lithium ion secondary cells to be used as drive power sources for vehicles, there is durability (cycle characteristic), that is, the ability to maintain good cell performance even in a usage state in which high-rate charging and discharging are repeatedly performed over a long period of time.
Research and development have been heretofore conducted from various angles to improve the durability. One of approaches involved the investigation of configuration (composition) and structure of positive electrode active materials to be used in the lithium ion secondary cells.
For example, as an example of the such a technique, Japanese Patent Application Publication No. 2001-266879 discloses a positive electrode active material (surface-coated positive electrode active material), other than a Co-containing active material, characterized in that the surface of the positive electrode active material is coated with a predetermined electrically conductive oxide, this positive electrode active material being an alternative to Co-containing active materials. Japanese Patent Application Publication No. 2001-266879 indicates that by providing such a coating, it is possible to improve the cycle characteristic over that of a positive electrode active material in a non-coated state.
Further, Journal of Power Sources, Vol. 259, pp 188-194, 2014, discloses, as one of surface-coated positive electrode active materials, a surface-coated positive electrode active material in which the surface of a positive electrode active material made of a LiNi0.5Mn1.4O4 with a spinel structure is coated with YBaCu3O7 which is a superconductor. It is also indicated that such a coating makes it possible to suppress the outflow of a transition metal from the positive electrode active material placed in a nonaqueous electrolytic solution.